There are a number of surgical procedures in which a fastener such as a screw or nail is inserted into a tissue of a patient. One surgical use involves insertion of an interference screw into a bone tunnel or channel to secure a bone plug in place and thereby attach an end of an anterior cruciate ligament (ACL) replacement. ACL reconstruction procedures and interference screws are disclosed for example in U.S. Pat. Nos. 5,139,499 and 5,139,520, both incorporated herein by reference.
Surgeons and patients desire absorbable implants which will be partially or completely absorbed by the body. An absorbable lag screw for fracture fixation and having an enlarged, rounded head, for example, is disclosed in German Patent DE 3,811,345.
A relatively large amount of torque must be applied to an interference screw during insertion. A screw formed from a bioabsorbable material is likely to have a significantly lower strength, however, and therefore should not be subjected to high torque during insertion. The distal region of a bioabsorbable screw is particularly susceptible to shear failure due to excess torque. It would be desirable to improve the configuration of the screw or to lower the amount of torque which must be applied to the screw.
Another type of interference device has annular ribs or ridges and is linearly inserted into a hole drilled in a bone. As disclosed in European Patent No. 317,406, for example, a conical peg having a plurality of annular ridges is driven linearly into a bone tunnel between the tunnel wall and a replacement ligament.
It is generally more difficult to precisely control the depth of insertion of such a linearly inserted implant and to control the amount of frictional engagement between the implant and the bone. Further, the tunnel walls are scraped or shaved to some extent by the annular ridges which may generate undesirable debris and expand the tunnel to the greatest outer diameter of the implant. Additionally, a bone plug within the bone tunnel might be unintentionally driven deeper into the tunnel.